The invention relates to a headbox of a paper machine for producing paper, board, tissue etc., and particularly relates to a process for interference free charging of the headbox with paper stock suspension and auxiliary materials.
Headboxes in paper machines receive paper stock suspension, which is fed to them through a pipeline, distribute the suspension uniformly over the headbox width and discharge the distributed suspension onto a dewatering wire of a Fourdriniere wire or hybrid former, or onto two dewatering wires of a double-wire former, in the form of a machine-width jet. The uniformity of the distributed suspension relates both to the mass distribution of the solids contained in the suspension over the stock jet width across the width of the headbox and over the stock jet height and also to the velocity distribution of the suspension over the width of the stock jet. As to the latter factor, a localized change in suspension velocity at a width location could locally affect the fiber orientation in the paper produced in the machine particularly along the interfaces in the web of paper between the localized region where the suspension had changed velocity and adjacent regions where the suspension had not similarly changed velocity.
If the foregoing distribution tasks are not fulfilled, then the paper quality, such as the mass per unit area distribution over the web width, that is, the mass per unit area transverse profile and/or a pre-set fiber orientation transverse profile, are disturbed.
In order to fulfill the distribution tasks, headboxes have various flow sections. The suspension is fed from a pipeline to a transverse distribution pipe that runs over the width of the headbox. This pipe has a flow cross section that decreases in the flow direction of the pipe across the width of the headbox in order to even out and control the suspension over the width. For example, the velocity and force of the suspension being fed from the pipe into the headbox may be made uniform across the width.
The transverse distribution pipe is joined to one or two guide devices within the headbox and the pipe, and the guide devices are typically separated by an intermediate channel or chamber from the distribution pipe. The guide devices generate turbulence, align the flow and provide uniform outflow from the downstream nozzle which follows the guide devices. The nozzle tapers narrower in the flow direction. The downstream end of the headbox has a machine width nozzle gap, from which the stock jet emerges in the direction of the web former.
Even with an optimal headbox configuration, interfering variables act on the paper manufacturing process and disturb the mass per unit area distribution, for example. Many headboxes therefore have a slice at the nozzle gap, which enables local setting of the gap width, which here means the local height of the outlet opening, in order to correct the mass per unit area over the paper web width.
DE 40 19 593 A1, which corresponds to U.S. application Ser. No. 08/662,980, incorporated herein by reference, discloses a new headbox principle in which the correction of the mass per unit area distribution in the paper produced by the machine including the headbox is carried out by locally changing the consistency of the pulp suspension at locations in the headbox. In this case, the feed to the headbox, viewed over the width of the headbox, is formed by a large number of separate channels, so called sections. A suspension mixer is connected upstream of each section. Two partial flows are fed to each mixer where they are mixed to form a mixed volume flow or section volume flow. The first partial flow is comprised of paper stock suspension having a solids concentration C.sub.H. The second partial flow is comprised of water, or preferably wire water or white water from the paper manufacturing process, having a solids concentration C.sub.L, wherein the concentration C.sub.L is smaller than the concentration C.sub.H. The arrangement enables the mixture ratio of the two partial flows to be set in a deliberate manner, without changing the total, combined sectional mixed volume flow at each section, i.e., without changing the velocity of flow at each section. This has the advantage that the fiber orientation transverse profile of the paper produced is set in the particular section or being set in adjacent sections is not impaired by a local area flow velocity change during the local correction of weight per unit area.
As a result of the development of these so-called dilution water headboxes, it has been possible to improve paper quality significantly, in terms of the quality of the mass per unit area transverse profile and the fiber orientation transverse profile. However, increasing paper machine operating speeds make it more difficult to achieve constant, respectively desired conditions for good paper quality in the paper manufacturing process. Interfering influences become larger. At the same time, the requirements of the converter as to various paper properties, such as printability, strength relationships and optical properties, are increasing. Defined properties over the paper web width and paper web thickness are particularly important.
In the forming area of the machine, small differences in the condition of the wires and of the dewatering elements have an increasing interference effect over the width at increasing paper machine speeds. This can produce differences in the dewatering and thus in the retention of the various solids materials contained in the paper suspension over the width, and can thus produce a different composition of the finished paper web. This leads to a streaky distribution of the paper properties over the web width.
EP Publication 0 651 092 A1 discloses a multilayer headbox for deliberately influencing the distribution of fillers and chemicals over the paper thickness, that is over several layers in the z-direction. Each layer has its own feed which passes separately from the other layers within the headbox. Metering points for chemicals and fillers are provided in the respective feeds. This enables manufacture of papers with different compositions over several layers in the z-direction.
However, this solution is very complicated, as compared with a single layer headbox, because separating lamellae are required in the nozzle and because at least three feed systems are used, i.e., usually one for each layer. A further disadvantage is that the auxiliary material or fillers and chemicals distribution can be influenced only in the z-direction and not in the transverse or width direction, i.e., the y-direction.
Thus, streaks occurring over the width cannot be prevented.
U.S. Pat. No. 5,560,807 discloses a headbox in which it is possible to influence the fillers and chemicals distributions in both the z- and the y-directions. In this case, the metering lines for auxiliary materials open into the transverse distributor in rows between the pipe openings of the pipes of the guide device. The direction of the metered flows is counter to the machine running direction and is at 90.degree. to the feed direction of the main flow in the transverse distributor pipe. A metered flow is therefore intended to be carried downstream by the main flow and to be carried by the main flow into the adjacent pipe of the guide device, for example, to influence the filler content at the point in the paper that aligns with the corresponding pipe.
The inflow from the metering lines to the distribution pipe has a disadvantageous effect in this arrangement. For example, if it is intended to correct the filler transverse profile, then the appropriate quantity of filler must be brought to the correct point along the y-direction. If the amount of filler, that is, the metering volume flow, is increased, then the inflow velocity of the filler necessarily increases. The metering stream penetrates more deeply into the main flow and is consequently swept further downstream along the path of the main flow. As the metered amount increases, this presents a risk that filler will be supplied, not to the adjacent pipe of the guide device as intended, but instead to the next further away pipe. This would influence the suspension at the wrong point across the headbox and would worsen the filler profile in the y-direction of the paper produced.
A paper grade change presents a particular problem for maintaining a predetermined profile, since it is often accompanied by a change of the overall flow volume. Values from experience show that the ratio between the maximum and minimum throughput may be 2 to 3. This means that the velocity in the transverse flow distributor for paper grade A may be three times the velocity for grade B. This likewise leads to the above described dragging of the metered substances in the y-direction.
A further solution for metering additives into a headbox is proposed in German application 196 32 673.7, dated Aug. 14, 1996. Metering, for example, is done in the area of the transverse distribution pipe, or in the pipes of the guide device or in the outlet nozzle. The disadvantages described above also occur with these solutions. In addition, metering into the pipes of the guide device is very complicated in terms of production, particularly where there are a large number of rows of pipes, which are often offset in relation to each other. Further, metering is barely possible because of the small size of the metering pipe cross sections. Metering the additives into the nozzle space in this manner can lead to streak formation of the additives, since no guide device with significant mixing turbulence follows. A further disadvantage resides in the risk of fiber string formation at the lance like metering pipes, which penetrate at right angles to the main flow.